The present invention relates generally to techniques for selectively growing diamond in conjunction with silicon, quartz, sapphire, silicon oxide, silicon nitride, silicon carbide or other medium to high temperature material suitable for diamond growth. It provides a broad process solution for any system in which a diamond mold can be formed and separated by materials suitable as etch resists to remove any portion of the mold structure. In particular it relates to integrating diamond tip structures on cantilevers for Scanning Probe Microscopy in processes easily manufactured using conventional equipment and process techniques which can be used to make high yield wafer quantities of such integrated tip structures. This technique is related to that earlier disclosed in the above-referenced U.S. patent application Ser. No. 11/048,611 (now U.S. Pat. No. 7,323,111) in which the separated tip structure is further bonded to a separate silicon or other material or MEMS structure.
The present invention relates more specifically to techniques for preparing a silicon on insulator (“SOI”) wafer so that diamond may be grown into a mold on the device layer or handle layer which and after growth the portion of the device which is to be exposed as a tip or other diamond structure is exposed by removal of the surrounding mold material.
The primary embodiment is an AFM Guided Nanomachining or indentation tip. The principal elements of the device consist of an SOI wafer on which a series of squares have been patterned along with cantilever structures and other marks and elements. Typically this pattern is on the device layer of an SOI wafer, after etching the exposed silicon oxide in the square hole is removed and the a wet etch is used in the exposed square pit to etch a self limiting four sided pyramidal recess ending in a point or straight edge depending on the crystal orientation of the silicon and/or precision of the square. Thus on 100 silicon a perfect square self-limits in a wet etch to a point, and a rectangle self-limits to a straight edge whose length depends or the ratio of the starting rectangles sides. Off axis silicon produces a point and a tilted pyramid with the proper rectangle. After the tip structure is formed additional MEMS structures may be added to the diamond and/or silicon (or other material such as sapphire, or silicon carbide) by silicon oxide bonding, metallic bonding or the use of adhesives to bond to any other material including silicon, sapphire, glass, fused or single crystal silica, plastics or metals.
In another embodiment the device is made from a sapphire or silicon carbide wafer which has been coated with polysilicon and then silicon oxide bonded to a silicon wafer after which the sapphire is reduced to a proper thickness for the MEMS function (typically a 2 to 50 micron thickness for SPM cantilevers) and the process herein made by patterning and etching the sapphire or silicon carbide, dry etching through the polysilicon, removal of the silicon oxide followed by wet etching of the self terminating pyramidal pit in the silicon wafer. The wafer is then processed to grow the diamond as described elsewhere. This process has the advantage that the end product can be readily metal bonded to any substrate including glass, quartz, silicon, or any metal (with a reasonable melting point at or above 100° C.). Further the mold material may be wet etched with common means to remove the mold silicon, polysilicon and silicon oxide without concern for damage to the MEMS (typically cantilever) structure. Further the polysilicon can be separately patterned for breakaway structures protected by silicon dioxide while the main mold silicon material can be removed by an etch which is preferential for silicon.
In a further embodiment the wafer structure as above can be coated by a layer of metal or other material such as a PTFE, epoxy or plastic after the silicon mold wafer is removed and used in a CMP, thermal annealing, or other process provide a final finish on the device side in which the thick coating has filled the diamond coated pit forming the inside of the molded diamond part such that a good optical surface is formed over the diamond structure. This can be particularly important for the manufacture of reliable SPM tips.
In yet another embodiment a secondary wafer or individual structure is bonded by any means to cantilever backside to form an optically smooth surface and increase the cantilever thickness and resonant frequency.
In a further embodiment the finished structure and in particular a cantilever is bonded to a substrate for transport and handling purposes by bonding to any of a glass, quartz, plastic, silicon, sapphire, machinable ceramic, ceramic, metal or other suitable substrate material prepared for each individual cantilever, small group or the entire wafer full of cantilevers.
In another embodiment the structure (particularly low force AFM cantilevers) is made from strong and flexible material such as silicon nitride or silicon carbide, diamond, diamond like carbon, carbon nanotubes, inert metals such as gold, palladium, platinum or rhodium. As above this structure with diamond molded part may be overcoated and/or CMP'd before the mold release etch and bonded to a suitable substrate. For example a low force classic double arm contact cantilever with diamond tip for AFM applications may be made by etching a 100 to 200 nanometer low stress silicon nitride film deposited on a single side polished silicon wafer. The etch pattern includes a square region on the end of the cantilever, two arms joined to make a triangle cantilever attached to a large rectangular body of silicon nitride which will subsequently be attached to a substrate.
After etch the wafer is spin coated with resist, pits are etched in the now exposed squares only to form pyramidal depressions and a seeding process (described elsewhere in more detail) consisting of either or both an immersion in an ultrasonic bath with nanosized diamond particles and sonicated for some time and/or exposure to a carbon plasma in high vacuum to coat exposed squares. The resist is stripped away and the diamond growth process proceeds filling the pits and leaving a small region around the silicon nitride square evenly coated with diamond. A silicon dioxide overcoat is then made and the parts bonded to a quartz wafer which is later patterned and etched to release the individual parts or at least the parts and a breakaway structure for easily removing the part from the wafer array. Furthermore in this example before the silicon dioxide coat a thick (1 to 2 micron) coating of silicon nitride is laid down over the cantilevers and diamond tips in molds filling any cavity in the molded diamond pyramid. The wafer is then polished on the silicon nitride side down to or nearly to the original silicon nitride film.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.